An optical system having a plurality of light sources to irradiate an optical disk for writing and reading of data is conventional. However, there are different types of optical disks that operate under different standards such as CD-ROM, CD-R (W), DVD each of which relate to a different type of media and, as such, each disk operates at a different wavelength for writing and reading of data. To accommodate these different wavelengths, an optical disk driving apparatus needs to install an optical pickup device having a light source that can output a laser beam of a wavelength suited to each type of media. A single optical disk driving apparatus that can operate under all of the different media standards with each type of optical disk employing a common optical pickup device to reduce the cost of additional equipment has been proposed. Such an optical pickup device should accommodate the different light sources and should output a laser beam of a different wavelength along a common optical laser beam path in which the light source outputs merge.
Fine tuning of the light intensity from each of the laser beams before irradiation onto an optical disk is essential to accurately image the laser beams on the optical disk, in order to flawlessly record signal data thereon, and to flawlessly reproduce data therefrom. However, it is inevitable that surrounding conditions such as temperature and the like will cause the output from each of the laser light sources to fluctuate thereby varying the intensity of the laser beams irradiated onto the optical disk. As a countermeasure, a photodetection element may be provided to monitor each laser beam output from each laser light source for regulating the coulombs of current flowing into each laser light source based on the monitoring result. Further, the condition of the optical disk or the condition of the signals recorded onto the disk may affect optimal light intensity values. In this situation, an optical pickup device should be used capable of independently controlling each of the laser light sources.
An optical system using a pickup device of conventional technology for irradiating laser beams onto an optical disk is shown in FIGS. 5A and 5B wherein divergent light is emitted from the semiconductor laser light sources 1, 2 as parallel light beams which are directed through a collimating lens 4 via a coupling prism 3 and reflected by a raised mirror 5 through an objective lens 6 onto an optical disk 10 (shown in FIG. 5B).
Photodetection elements 7, 8 are arranged in the vicinity of the laser light sources 1, 2 to detect luminous energy from divergent light emitted from the laser beams. The laser beams may directly reach the photodetection elements 7, 8, or they may be indirectly guided by a raised mirror or the like.
The photodetection elements 7, 8 in the optical pickup device detect luminous energy from each laser beam in proportion to the intensity of the laser beam. Accordingly, current flowing into the semiconductor laser light sources 1, 2 may be regulated based on the output from photodetection elements 7, 8, for controlling the intensity of the laser beam output of each light source.
Nonetheless, the configuration of FIGS. 5A and 5B have a problem in that for every laser beam of a different wavelength a separate photodetection element is required, thereby increasing manufacturing costs and decreasing the available space in an optical pickup to accommodate the multiple photodetection elements.
To overcome this drawback, one may consider a pickup device in which a single photodetection element 9 is used and arranged in the vicinity of the optical path of the laser beam after passing through coupling prism 3 as is illustrated in FIG. 6. Currently, the output of a laser diode for a CD system is at least 200 mW or over and the output of a laser diode for a DVD system is 100 mW or less. In addition the photodetection element for a CD system has its sensitivity peak near 780 nm, which levels off about 20% i.e. its sensitivity drops off 20% for a DVD system which utilizes a wavelength of, for example, 650 nm. It is, therefore, difficult for a single photodetector 9 alone to be used in the configuration of FIG. 6 to detect the luminous energy of laser beams having different wavelengths and to control the current level flowing into both semiconductor laser diodes 1 and 2.
The present invention overcomes the problems of using a single photodetector and provides an optical pickup device capable of controlling multiple laser light sources without space or arrangement limitations. Moreover, the photodetector of the present invention can automatically regulate the sensitivity of a selected laser beam of given wavelength spectra, eliminating the need for multiple photodetectors. Consequently the need for a control circuit in the external control system and the need for a cable for signal control are also eliminated, further allowing a reduction of manufacturing costs.